The Truth BEhinD the Soldier......

   Although not much is known about his past, based on Episode Zero, Heero was raised by an assassin named Odin Lowe. This is the same man who murdered the late president Heero Yuy. During a mission, Odin unfortunately dies and Heero is left to finish it. Heero is soon discovered by Doctor J and thus, his involvement with operation meteor begins.

             If you're all wondering why Heero is emotionless, this spoiler could probably explain why [taken from Endless Waltz]: Heero ran into a little girl and her dog after planting bombs in an OZ base. She gave
him a yellow flower and ran off. When it was time to detonate the bombs, he unfortunately made some miscalculations and a Leo accidentally crashed some neighboring apartment buildings. The next day, Heero discovered the body of the girl's little dog and a teddy bear (that bear in the op song). Comepletely devastated, Heero picked up the dog and returned to base where Dekim Barton  tells Dr J to retrain him. Why? "There is no room for human sympathy in war".

If u Gundam WIng/ Heero Fans didn't know... Heero Yuy is just a code name for our "antisocial" one.  His real name is Odin Lowe Jr...  If that doesn't ring a bell look above and that should help ya..